1.) If I'm using v. 0.90 as my offline wallet and don't want to upgrade, do I need to use an older version of Armory for my online wallet?
0.91 at best. You should upgrade, at least to get the security and bug fixes.
Not being hosted officially anymore. This is a ATI release, so you are looking at the right place.
ATI releases are hosted on bitcoinarmory, the community page built around my fork is on btcarmory. ATI last version is 0.93.3. They do link to my release page however.
I find it very confusing that there are two sites where you can download wallets given the security risks involved.
The security of binary distribution is provided by the offline signature on packages, not where you get them from.